A number of synthetic polymeric materials, which contain acidic groups which impart anionic functionality thereto are known in the art; possibly the most widely available and best known of such anionic synthetic resins, are the cation exchange resins available under such trade-names as Amberlite, Dowex, Permutit and Zeocarb. In general the so-called "weak" cation exchange resins contain carboxylic groups while the so-called "strong" cation exchange resins contain sulfonic groups. However, such cation exchange resins are not hydrogels.
Other anionic synthetic polymeric materials which are known in the prior art, are the self-stabilizing polymer latices obtained by emulsion polymerization techniques in which a copolymerizable surfactant is used as an emulsifier in the preparation of the aqueous emulsion of monomer(s) to be polymerized. In the course of the polymerization, these copolymerizable surfactants copolymerized with the monomer or mixture of other monomers being polymerized and become an integral part of the resulting polymer so that the polymeric material so obtained contains ionic (acidic) groups. As examples of acidic, ionic, copolymerizable surfactants which have been so used may be mentioned the polymerizable .alpha.-methylene carboxylic acid esters (e.g., the acrylic and methacrylic acid esters) of hydroxyalkane sulfonic acids such as those disclosed in U.S. Pat. Nos. 3,024,221 and 3,033,833 both to Le Fevre and Sheetz and U.S. Pat. No. 3,617,368 to Gibbs and Wessling; also the sulfate esters of hydroxyalkyl acrylates and methacrylates disclosed in my U.S. Pat. No. 3,839,393 issued Oct. 1, 1974; also the phosphate esters of hydroxyalkyl acrylates and methacrylates disclosed in my U.S. Pat. No. 3,855,364 issued Dec. 17, 1974; and the sulfates of polymerizable enthylenically unsaturated alcohols and their alkylene oxide adducts disclosed in my application Ser. No. 321,228, filed Jan. 5, 1973, now U.S. Pat. No. 3,875,202 issued Apr. 1, 1975. Such copolymerizable surfactants are also used to impart hydrophilic properties to the resulting polymer, to improve the receptivity of the resulting polymer to basic dyes and other purposes more fully described in the above patents; however, none of the polymers heretofore produced by their use have, to the best of my knowledge, been in the form of hydrogels.
Various synthetic polymeric hydrogels, including hydrogels produced by copolymerization of a mixture of monomers containing a hydroxyalkyl acrylate or methacrylate and a cross-linking agent, have also been disclosed in the prior art. As examples of cross-linked polymeric hydrogels, in the preparation of which a hydroxyalkyl acrylate or methacrylate and a cross-linking agent has been used, may be mentioned the cross-linked hydrogels disclosed in my prior U.S. Pat. No. 3,532,679 issued Oct. 6, 1974 and my copending application Ser. No. 383,275 filed July 27, 1973, now U.S. Pat. No. 3,878,175 issued Apr. 15, 1975.
While most of the known cross-linked hydrogels are neutral hydrogels and are not ionic in character, there is in U.S. Pat. No. 3,689,634, issued Sept. 5, 1972 to Kliment, Vacik, Majkus and Wichterle, entitled Protracted Activity Oral Hydrogel Bead; a broad suggestion that "it is also possible to replace the non-ionizable cross-linked hydrogels by physically similar hydrogels containing also ionizable groups"; the only examples of ionic hydrogels disclosed in this patent are: "A porous hydrogel capable of exchanging cations prepared by copolymerizing a mixture of 35 parts of methacrylic acid, and 30 parts of a 25 percent aqueous solution of maleic anhydride," disclosed in Example 8 at the top of column 8 of the patent; and "A copolymer prepared from 97 parts of ethylene glycol monomethacrylate, 2 parts of methacrylic acid and 1 percent of ethylene glycol bis-methacrylate by suspension polymerization in a concentrated, aqueous solution of sodium chloride, using 0.05 parts of diisopropyl percarbonate as a polymerization initiator," disclosed in Example 9 at the middle of column 8 of the patent. The acid groups in such anionic hydrogels disclosed in this patent are of course carboxylic acid groups. As in the case of cationic exchange resins containing carboxylic acid groups which are classified as "weak" cation exchange resins, such carboxylic hydrogels are relatively weak cation exchangers.